This invention relates to radiation imaging systems and, more particularly, to an imaging system employing a radiation scanner with an array of infrared detectors in combination with averaging and adaptive signal compression circuitry for improved uniformity and clarity in reproduction of scenes to be imaged by the system.
One well known form of infrared imaging system employs an array of infrared detectors and a lens for directing rays of radiation from a portion of a scene being viewed to the array of detectors. Typically, the detectors are arranged along a line and are scanned in a direction perpendicular to the line, the line of detectors providing the width of a swath being swept during a single line of scan.
In one application of considerable interest, the scene being imaged is viewed on a display wherein the scan lines are parallel to a horizontal axis of the display while the line of detectors is parallel to a vertical axis of the display. Typical subject matter of the foregoing scene would include both sky and land. As viewed by infrared detectors, the sky presents subject matter which is of a lower temperature than the subject matter presented by land. Also included within the typical scene would be objects of higher temperature, such as aircraft, within the sky, and cooler subject matter such as a lake or river on the land. Buildings, roadways, trees, and other foliage may also be present as further subject matter within the scene.
Subject matter in the foregoing portions of the scene emit infrared radiation at differing temperatures characteristic of the subject matter in response to illumination by the sun, or in response to some other source of heat. Each detector produces an electric signal of varying amplitude dependent upon the temperature of the subject matter of the portion of the scene being viewed by the detector. The signals produced by the array of detectors are processed in individual detector channels and may be applied to a display for presentation of an image of the various objects in the scene being viewed.
A problem arises in that variations in temperature from the coldest subject matter of a scan line to the hottest subject matter of a scan line would produce a temperature differential which exceeds the dynamic range of the display. As a result, in the use of scanning equipment wherein all detector channels are subject to a common gain control, a choice must be made as to which part of the scene is to be presented on the display. If the gain is increased for viewing weak signals from cool subjects, the strong signals from hot subjects may saturate the display and cause streaking. Alternatively, if the gain is reduced, the cool subjects may be lost from the display.
A further problem arises in the operation of such a scanner due to a variation among the detectors in their responsivity to incident infrared radiation. Such variation in responsivity, if uncorrected, introduces a lack of uniformity to the displayed image. Thus, two detectors receiving the same radiation may produce output signals of differing amplitudes. This further compounds the foregoing problem of the dynamic range in that portions of the image scanned by certain ones of the detectors may be overly intense or excessively weak in their presentation on the display. The lack of uniformity degrades the displayed image quality and obscures information in the image.
One solution to the problem of reduced uniformity due to differing detector responsivity has been the use of individual alignment circuits incorporating manually adjustable potentiometers for each of the detectors. The solution is disadvantageous because of a requirement for excessive operator time in the manual adjustment of the potentiometers in a scanner of many, possibly in excess of one hundred, detectors in the detector array.
In the construction of the typical scanner, all of the detectors are placed within a single chamber which is maintained by cooling equipment at a low temperature. When replacement of a number of the detectors is required, the entire array is replaced in which case all of the potentiometers must be reset manually for alignment of the scanner.
A further solution to the uniformity problem has been the scaling of all signals presented to the display as by use of automatic gain control employing the injection of a pilot signal into the field of view of each of the detectors. Such operation involves unwanted complexity such as additional optics for injection of an infrared pilot signal.
The foregoing attempts to solve the uniformity problem do not treat the foregoing limitation on the dynamic range of signals which can be displayed.
Yet another problem arises in the use of infrared scanning for producing a scene image, this problem being a blurring of edge lines of subject matter by components of the scanner optical system. Such blurring may be considered as the convolution of the scene image with a blurring function resulting in a loss of definition at the high-frequency portion of the spectrum of each scanned detector signal. Data in the middle and the low frequency portions of the spectrum are not significantly affected by such blurring.